In general, power conversion involves converting electric energy from one form to another. For example, power conversion may include converting between alternating current (AC) and direct current (DC), changing from one voltage or frequency to another, or some combination of these. As a specific example, an AC-AC converter converts an AC waveform to another AC waveform. Typical converters require numerous switches and are therefore complicated and expensive.
Partial resonant converters have several advantages over the other types of converters including cycloconverters, matrix converters, resonant converters and DC link converters. This universal power converter is a suitable candidate for variety of applications as it can overcome most of the problems associated with the other types of converters.
In partial resonant converters that are categorized as AC-link converters, the link frequency can be as high as allowed by the switches and control processor. Such converters are in fact AC-AC, DC-AC, AC-DC, or DC-DC buck-boost converters with an alternating inductor current and voltage. Therefore, unlike cycloconverters and matrix converters, this converter is capable of both stepping up and stepping down the voltage and frequency. Alternating inductor current and voltage, which maximizes the inductor/capacitor utilization, is achievable through bi-directional switches.
Partial resonant AC link converters, also called AC-link universal power converters, are compact, reliable, and efficient and offer longer life times compared to other types of converters. However, partial resonant AC link converters typically require numerous switches, which may require complicated controls and processing. For example, FIG. 1 is a circuit diagram of a typical partial resonant converter 100 disclosed in U.S. Pat. No. 7,599,196 requires 12 bidirectional-conducting-bidirectional-blocking switches which are usually formed by 24 forward-conducting-bidirectional-blocking switches or 24 bidirectional-conducting-forward-blocking switches (S0 through S23).
The switches (S0 through S23) in the partial resonant converter 100 turn on at zero voltage and have a soft turn off. Therefore, the voltage and current stress over the switches are reduced and the switches have negligible switching losses. Since the switching losses are negligible, the switching frequency and consequently the link frequency can be very high. Compared to DC-link converters, the size and reliability of this converter is improved while offering a longer lifetime. The control algorithm in this converter guarantees the isolation of the input and output; however, if galvanic isolation is required, a single-phase high frequency transformer may be added to the link.
The partial resonant converter 100 transfers power entirely through the link inductor L which is charged through the input phases and then discharged into the output phases. The frequency of the charge/discharge is called the link frequency and is typically much higher than the input/output line frequency. Between each charging and discharging there is a resonating mode during which none of the switches conduct and the LC link resonates to facilitate the soft switching. Charging and discharging of the LC link in a reverse direction is feasible through complimentary switches located at each leg which leads to an alternating current in the link.
The resulting input and output current pulses should be precisely modulated such that when filtered, they achieve unity (or desired) power factor at the inputs while meeting the output references. In an AC-AC converter, there are three input phases and one link to be charged through these input phases. In order to have more control on the input currents, close to unity or desired PF at input, and minimized input and output harmonics, the link charging mode is split into two modes. Similarly link discharging can be split into two modes. Again between each charging or discharging mode there is a resonating mode which facilitates zero voltage turn on. In other words, charging is done via two input phase pairs (during modes 1 and 3) which are nominally the lines having the highest and the second highest instantaneous voltages (for unity power factor). The charged link discharges into two output phase pairs similar to the charging process.
The basic operating modes and relevant converter waveforms for the three-phase AC-AC conversion case are represented in FIGS. 2A-2F and FIG. 3, respectively. Each link cycle is divided into 16 modes, with 8 power transfer modes and 8 partial resonant modes taking place alternately. The link is energized from the inputs during modes 1, 3, 9 and 11 and is de-energized to the outputs during modes 5, 7, 13 and 15.
Mode 1 (Energizing) is shown in FIGS. 2A and 3. Before the start of mode 1, the incoming switches, input switches which are supposed to conduct during modes 1 and 3, are fired (S6, S10 and S11), however they do not conduct since they are reverse biased. Once the link voltage, which is resonating before mode 1, becomes equal to the maximum input line-to-line voltage (VAB), proper switches (S6 and S10) are forward biased initiating mode 1. This implies that the turn on occurs at zero voltage as the switches transition from reverse to forward bias. Therefore, the link is connected to VAB_i via switches which charge it in the positive direction. The link charges until phase B input current averaged over a cycle time, meets its reference value. Switch S10 is then turned off.
Mode 2 (Partial Resonance) is shown in FIGS. 2B and 3. During this mode none of the switches conduct and the link resonates until its voltage becomes equal to that of the input phase pair that is supposed to charge the link during mode 3 (VAC_i). This is the phase pair the link charges next from. In this mode, the circuit behaves as a simple LC circuit.
Mode 3 (Energizing) is shown in FIGS. 2C and 3. Once the link voltage is equal to the voltage across the input phases AC_i, switches S6 and S11 are forward biased initiating mode 3, during which the link continues charging in the positive direction from VAC_i.
Mode 4 (Partial Resonance) is shown in FIGS. 2B and 3. During mode 4, the behavior of the circuit is similar to that of mode 2 and the link voltage decreases until it reaches zero. At this point the incoming cells, the output switches that are supposed to conduct during modes 5 and 7 (S19, S20 and S21 in FIGS. 5 and 6) are turned on; however being reverse biased they do not conduct. Once the link voltage reaches VAC_O (Assuming |VAC_O| is smaller than |VAB_O|) switches S21 and S20 will be forward biased and they start to conduct initiating mode 5.
Mode 5 (De-energizing) is shown in FIGS. 2D and 3. The output switches (S20 and S21) are turned on at zero voltage to allow the link to discharge to the chosen phase pair until phase C output current averaged over the cycle meets its reference. At this point S20 will be turned off initiating another resonating mode.
Mode 6 (Partial Resonance) is shown in FIGS. 2B and 3. The link is allowed to swing to the voltage of the other output phase pair chosen during Mode 4 (e.g., swings from VAC_O to VAB_O).
Mode 7 (De-energizing) is shown in FIGS. 2E and 3. The link discharges to the selected output phase pair until there is just sufficient energy left in the link to swing to a predetermined voltage (Vmax) which is slightly higher than the maximum input and output line to line voltages. At the end of mode 7 all the switches are turned off allowing the link to resonate during mode 8.
Mode 8 (Partial Resonance) is shown in FIGS. 2B and 3. The link voltage swings to −Vmax and then it starts to decrease.
Modes 9 through 16 are similar to modes 1 through 8, except that the link charges and discharges in the reverse direction. See e.g., mode 9 shown in FIG. 2F. For this, the complimentary switch in each leg is switched when compared to the ones switched during modes 1 through 8.
FIG. 4 illustrates the link current, link voltage, current passing through switch S20 and current passing through switch S17 in the partial resonant converter 100. FIGS. 5 and 6 illustrate filtered input and output currents of the partial resonant converter of FIG. 1.